Booting off raidz *mostly* work

[Artem Belevich]

> I'm not sure we can use the grub's code directly...

Probably not -- it's under GPL. Besides, it does not seem to support
raidz/raidz2.

> How could we verify whether it is gang blocks that break the loader?

Beats me at the moment. Perhaps we could use zdb (is there any
documentation on it?) to dump info about all blocks used by files
involved in booting and then see if any of  them are gang blocks.

--Artem